The present studies address a previously undescribed mechanism of platelet activation by the yeast phase of H. capsulatum. To date it has been established that this reaction requires prior coating of the yeasts by gamma globulin and, if aggregation is to be observed by fibrinogen. The direction now is a further understanding of the role of gamma globulin in this interaction, investigating whether this is a function of specific antibody, the number of combining sites involved, the reactive site of the IgG molecule in its combination with H. capsulatum (FAb or Fc), the role of the Fc fragment in the interaction of the H. capsulatum-IgG complex with the platelet membrane, and blocking of various phases of the reaction with intact and antibody fragments.